1. Field of the Invention
The present invention relates generally to Air Traffic Control (ATC) systems, and more particularly, to improvements in the air/ground data communication link provided by a Mode-S Secondary Surveillance Radar (SSR) beacon system.
2. Description of the Prior Art
Current radars for surveilling airborne aircraft evolved from designs developed for military use during World War II. The ATC Radar Beacon System (ATCRBS) is a secondary, or beacon radar that grew out of the Identification Friend or Foe (IFF) military system. ATCRBS is a cooperative radar in that it does not rely on the receipt of reflected energy from aircraft. Instead, the aircraft carries a transponder, i.e., a receiver/transmitter. The transponder recognizes interrogations from a ground based radar and transmits a reply. This capability greater increases the surveillance range of the radar and enables an aircraft identification function (called a Mode-A reply) wherein the transponder attaches an identification code to its reply. In addition to the identification function, many aircraft connect their altimeters to the transponders so that their replies can include altitude information (called a Mode-C reply). In both Mode-A and Mode-C systems, when transmitting information, an SSR sequentially transmits interrogation signals to aircraft in the area for requesting information from the aircraft. The interrogation signal transmitted by the SSR contains three pulses, with the first and third pulse being separated by a pre-determined width and transmitted at a specific frequency. The second pulse is a side-lobe suppression signal transmitted from an omnidirectional antenna co-located with a mechanically rotating antenna which provides a highly directive antenna beam in a horizontal plane. The time interval between first and third pulses defines what information the interrogator is requesting i.e., eight (8) microseconds for identification and twenty-one (21) microseconds for altitude. Upon receipt of the interrogation signal, the aircraft transponder develops a reply signal to supply to the transponder the requested information consisting of identification and/or altitude. The SSR processes the received signal, together with time of arrival range information, to develop a measurement of position for each responding aircraft. The Mode-C and Mode-A systems are unable to relay additional information or messages between the SSR and the interrogated aircraft, other than the forenoted identification and altitude information.
During the 1960's, ATCRBS began to overload because of increases in the number of aircraft, the percentage of aircraft that were equipped with transponders, and the number of ATCRBS radar installations. Due to this overload, the Mode-C and Mode-A systems developed significant amounts of interference and garble because many aircraft transponders within the main beam of the interrogating SSR would give a reply.
In recognition of this and other deficiencies in ATCRBS, the Mode Select (Mode-S) system was developed to allow the active transmission of messages or additional information by the SSR, as well as the incorporation of various techniques for substantially reducing transmission interference. The Mode-S sensor includes all the essential features of ATCRBS, and in addition includes individually timed and addressed interrogations to Mode-S transponders carried by the aircraft. Additionally, the rotating directive antenna system is of monopulse design. Thus, the Mode-S sensor will allow full surveillance in an integrated ATCRBS/Mode-S environment.
The Mode-S sensor produces an identity tag for aircraft in the surveillance area by message transmission using one of two techniques, thereby enabling subsequent discreet addressing of the aircraft. One technique is a Mode-S SQUITTER performed by the Mode-S transponder (in the aircraft) the other technique is a Mode-S ALL CALL, performed by the sensor (on the ground). During a Mode-S SQUITTER, the Mode-S transponder spontaneously and pseudo-randomly transmits (squits) once per second, on its own, a specific address code, unique to the aircraft carrying the transponder. During a Mode-S ALL CALL, the Mode-S sensor transmits an ATCRBS- like spatial identity interrogation signal which elicits a transponder reply transmission of discrete identification.
As noted above, the Mode-S transmissions have two different message lengths, one has 32 bits of data and the other has 88 bits of data. Additionally, each message includes 24 bits of Cyclic Redundant Cycle (CRC) error checking code bits, making the message lengths 56 bits and 112 bits in length, respectively.
Unfortunately, the format selected for Mode-S downlink transmissions is susceptible to multipath effects that produce delays in the order of 500 to 1,000 nanoseconds and oftentimes result in the delayed reflected signals being received with amplitudes comparable with those received by a direct path. However, in view of the fact that the Mode-S system includes a highly directive rotating antenna, the designers of the Mode-S system concluded that in the majority of cases, the reflected-path signal would either be much weaker or sufficiently delayed so as to be easily separable in the time domain from the direct-path signal. These designers visualized that the most prevalent interference problem would be the collision in time of a Mode-S reply with an ATCRBS signal. These signals are on the same frequency (1090 mHz) and transmitted at the same relative signal level. The technical solution provided by the designers was to include a cyclic redundancy check (CRC) parity word with each message transmitted. A 24 bit CRC was chosen, and can correct all error patterns not exceeding 24 bits in length. This works well to prevent ATCRBS interference because the ATCRBS message is less than24 microseconds in duration, and therefore corresponds to less than24 bits of the Mode-S reply length.
Described in much greater detail in the publication titled, MODE SELECT BEACON SYSTEM (MODE-S) SENSOR, available from the U.S. Department of Transportation, Federal Aviation Administration, Specification Number FAA-E-2716 amendment-2 dated Mar. 24, 1983, the Mode-S receiver includes a message processor which generates a "confidence string" to represent the quality of the received signal. In this case, quality means the cleanness of the bit-by-bit decisions provided by the message processor. The nature of the Mode-S transmitted message is that each data bit is one microsecond in length. A "one" is represented by a 1/2 microsecond pulse followed by a 1/2 microsecond space. A "zero" is represented by a 1/2 microsecond space followed by a 1/2 microsecond pulse. A confidence count of "1" indicates high quality and is generated when the 1/2 microsecond pulse amplitude is within a specific amplitude range relative to the preamble portion of the transmitted message, and the 1/2 microsecond space has no energy greater than a reference value of the pulse amplitude reference. The confidence count for each bit of the message is grouped together in a serial fashion with the confidence count for all the other bits of the message, to develop a " confidence count string." This confidence count string is further processed to determined that "0's" in the confidence string span no more that 24 consecutive data bit positions. If the "0's" span no more than 24 microseconds (24 consecutive data bit positions), the overall message is given a high "confidence count" or 1. If the "0's" span more than 24 microseconds, no attempt is made to decode/correct the message, and the message data is discarded.
An additional problem with the communication link provided by the Mode-S system is that there is increasing desire on the part of aircraft owners to have additional message capability between the aircraft and the ground. Typically, this additional information would include AOC (Aircraft Operational Command) information comprising two to three pages of text with flight arrival information, such as gates, passenger lists, meals on board, etc., as well as Flight Critical Data (FCD). Due to the nature of the Mode-S rotating antenna, the communication link is periodically broken, for example at four (4) second intervals, i.e., fifteen times per minute. Such periodic breaks in the communication link are extremely undesirable.
one solution to providing an improved Mode-S communication link is disclosed, for example, in U.S. Pat. No. 5,196,855 which discloses the use of an electronic scanning (E-scan) type of antenna in addition to the conventional highly directive rotating mechanical antenna, for increasing the data link communications capacity of the Mode-S sensor. A major disadvantage of this system is the expense and system complexity of the electronic scanning antenna and its support apparatus.
Another solution which would improve the communication link in a Mode-S system is to include an omnidirectional antenna for receiving the Mode-S transmissions such as described in an article titled "Propagation of Mode S Beacon Signals on the Airport Surface" published in The Lincoln Laboratory Journal, Volume 2, Number 3 (1989) pages 397-410. The use of omnidirectional antennas provides the advantage of lower costs as compared to an electronic scanning antenna and, if multiple omnidirectional antennas are used to provide multiple received sites, multilateration techniques can be used as a confirmation or "sanity" check of the aircraft position as determined by the conventional Mode-S processing, retransmitted GPS data, or other means.
Unfortunately, in a Mode-S downlink using an omnidirectional antenna, the transmissions are susceptible to multipath effects. As previously noted, conventional Mode-S receivers include a highly directional antenna having a sharply tapered lower edge of the receive beam. This tends to sharply reduce the amplitude of received reflected signals.
In an environment using multiple omnidirectional antennas for receiving the Mode-S transmission, as previously mentioned, there can be a significant amount of multipath effects where multiple signals are received with 500 to 1,000 nanosecond delay. Depending upon the consecutive bit pattern in the message, there is a strong likelihood that pulses will collide, due to the multipath, into the 1/2 microsecond spaces of the Mode-S message. This phenomena is code content dependent, as well as site dependent. In any event, the result of pulse collisions is that an error pattern will tend to distribute "0's" in the confidence string throughout the data message, with a span which will generally be greater than the 24 microsecond span that the CRC error correcting code is designed to handle.
It is an object of the present invention to provide an improved communication link for use with the Mode- S, or similar, System.
It is a further object of the invention to provide an SSR sensor which can provide not only high precision surveillance function, but also a high speed and large capacity air/ground datalink communications function.
It is a still further object of the invention to provide such improved data handling capacity at a relatively low cost, and furthermore in a manner which provides the ability to validate the Mode-S position information.